A traffic control system includes: for example, a central apparatus located in a traffic control center; and traffic signal controllers, vehicle detectors, information boards, traffic monitor terminals, and the like which communicate with the central apparatus via dedicated communication lines (refer to Patent Literature 1, for example).
In such a traffic control system, predetermined traffic indices are calculated on the basis of, for example, detection signals from vehicle detectors installed at appropriate locations in an area to be controlled, and traffic-actuated control, such as setting of optimum traffic light switching timings for a plurality of intersections, is performed on the basis of the calculated traffic indices.
As a vehicle detector for collecting data used for traffic signal control, a non-image-processing vehicle detector represented by an ultrasonic vehicle detector has been known. The non-image-processing vehicle detector performs spot measurement such as counting up the number of passing vehicles (traffic volume) through a relatively narrow detection spot (refer to Patent Literature 2, for example).
Meanwhile, as another vehicle detector for collecting the above data, an image-processing vehicle detector (television camera) has also been known. The image-processing vehicle detector has a photographing range including a relatively long road section, and digitally analyzes a photographed image of a vehicle to measure the speed of the vehicle, and the like (refer to Patent Literature 3, for example).